8 OF THE ASSIMILATION OF CARBON. 



ing and uncertain state of our knowledge on this subject up to 

 the present time, than that modern physiology has not kept pace 

 with the rapid progress of chemistry. 



In the following inquiry we shall suppose the humus of vege- 

 table physiologists to be really endowed with the properties 

 recognised by chemists in the brownish-black deposits obtained 

 by precipitating an alkaline decoction of mould or peat by means 

 of acids, and which they name humic acid. 



Humic acid, when first precipitated, is a flocculent substance, 

 is soluble in 2500 times its weight of water, and combines with 

 alkalies, forming with lime and magnesia compounds of the same 

 degree of solubility (Sprengel). 



Vegetable physiologists agree in the supposition that by the 

 aid of water humus is rendered capable of being absorhed by the 

 roots of plants. But according to the observation of chemists, 

 humic acid is soluble only when newly precipitated, and becomes 

 completely insoluble when dried in the air, or when exposed in 

 the moist state to the freezing temperature (Sprengel). 



Both the cold of winter and the heat of summer, therefore, 

 are destructive of the solubility of humic acid, and at the same 

 time of its capability of being assimilated by plants. So that, 

 if it is absorbed by plants, it must be in some altered form. 



The correctness of these observations is easily demonstrated 

 by treating a portion <-f good mould with cold water. The fluid 

 remains colorless, and is found to have dissolved less than 

 TToVoT P art °£ it- s weight of organic matters, and to contain 

 merely the salts which are present in rain-water. 



Decayed oak-wood, likewise, of which humic acid is the prin- 

 cipal constituent, was found by Berzelius to yield to cold water 

 only slight traces of soluble materials ; and I have myself veri- 

 fied this observation on the decayed wood of beech and fir. 



These facts, which show that humic acid, in its insoluble con- 

 dition, cannot serve for the nourishment of plants, have not 

 escaped the notice of physiologists ; and hence they have 

 assumed that the lime or the different alkalies found in the ashes 

 of vegetables, render soluble the humic acid, and fit it for the 

 process of assimilation. 



Alkalies and alkaline earths do exist in the different kinds of 



